Just a few hours ago Slashdot reported that a judge had refused an injunction against ReDigi, and now they are supposed to have won their case? I'd say there are two possibilities: One, that we have a judge who can run at speeds exceeding the speed of light, because that's the only way a case could have finished so quick. Or second, that the submitter is a clueless twat you didn't understand a word of what he is actually submitting. Since there is no link to any real information, I assume the latter.

True, but then if I sell my copy of a book there's no guarantee I haven't photocopied it and kept the copy either.

Bear in mind that it's not so much an argument over whether MP3s should be treated as material objects or not. It's that the record companies want them treated as material objects for purposes of one section of copyright law (Section 106 covering distribution of copies) but treated as not being material objects for purposes of a different section (Section 109 covering sale of copies). The counterargument is that the record company can't have it both ways, and the judge agreed that if the record companies want it to be considered a copy then defendants are entitled to treat it as a copy even when the record companies would rather they didn't.

I suppose that would be the difference between you doing something illegal and ReDigi (or another service) operating an illegal service.

Presumably what mattered here is that ReDigi did their due diligence in the process of transferring an mp3 from one person to another, destroying the provided original. If you copied it before that, then you did something illegal, not them. They'd have done what they could to avoid facilitating such a scam.

I agree but if I keep backups of all my music they're not going to be able to delete copies of music on a USB drive not even connected to the computer

I can buy a CD, create an image with EAC, then take the CD to any 2nd hand store and sell it to them, no question asked. Nobody makes you sign a document where you state that you have destroyed any backups you might have made. Heck, nobody even asks. Is it legal? Probably not. But that's not the point. Just because I can do this, I am not prohibited from selling said CD. So "possibly having backup copies left" is NOT an argument that can be used prohibiting someone from selling music files they bought from a legal source, e.g. iTMS. I can use my scanner to scan a BOOK. So with that reasoning, you can't sell books anymore because you might have kept a copy.

My concern would be as well is that someone decides to sell their low quality torrented music and you still have to worry about it being a quality version. Even if the 30 second preview is from the actual file it doesn't tell me if it cuts off too early.

Yes, that would be a problem. You never know if it might have been taken from the 2010 remastered edition where MC Master DJ Shitforface decided to master it really "hot" i.e. compressed to death and brickwalled. Perhaps these services should give you the option to listen to the first 5 seconds and the last 5 second, as well as supply all relevant information (including DR analysis, etc. which can all be automated).